From Act 1 of Neil Simon's Last of the Red Hot Lovers:
Barney: As a matter of fact, it's my mother's apartment.
Elaine: You're not actively serious?

I've never heard 'actively' in this context. My instinct would be to say 'actually'. So, is this 60s-lingo, an
Americanism, a typo, or what? Any thoughts out there?

Americanism, a typo, or what? Any thoughts out there?

Interesting usage. Not any Americanism this USn has ever heard. Seems a little odd for a typo. Simon giving a character a distinctive idiolect? I've known people whose use of the language was considerably different from the norm. Does this character have other odd speech habits?

There used to be spellcheckers that would correct typo, and substitute wrong word.
For instance, a couple years ago I saw in Smithsonian magazine "forward" where
"foreword" was clearly meant. I wrote, and got reply that spellchecker had done it.

spellcheckers that would correct typo, and substitute wrong word.

I had to spell it actielly for the Word2000 Spellchecker to correct it to actively.

Talking of spellcheckers reminded me of the editorial in yesterday's The Australian newspaper [as reported in the Media Watch TV program] in which the writer had meant to use the term "self-deprecating humour" but had somehow written "self-defecating". There must be an appropriate response to this but it's a bit too late at night to think of one.

"Self-defecating" humour: I was so funny I sh*t myself?

Simon giving a character a distinctive idiolect?

A good theory, Faldage, but other than this weird little 'actively', Elaine's speech is pretty ordinary. Oh well...

Oh well...

One of the minor mysteries of life. Was this from some transcript or an authorized holograph or what?

I'm playing Elaine, and it's in the script (Samuel French version). I might check another edition, just in case it was a poorly corrected typo. And if not, I'll just learn the line the way it is!

to me, "actively" serious has a slightly different meaning than (the overworked) "actually" - it seems to be saying that you are not only serious, but *doing something about it.

google gets a few hits, one of which is from your script.